1. Field of the Invention
The present invention relates to an improved electron beam collector and, more particularly, to a conduction cooled collector capable of highly depressed operation without voltage breakdown.
2. Description of Related Art
Many electronic devices employ a travelling stream of charged particles, such as electrons, formed into a beam as an essential function in the device's operation. In a linear beam device, an electron beam originating from an electron gun is caused to propagate through a tunnel, or drift tube, generally containing an RF interaction structure. Within the interaction structure, the beam must be focused by magnetic or electrostatic fields in order for it to be effectively transported through the interaction structure without energy loss. In the interaction structure, kinetic energy is transferred from the moving electrons of the beam to an electromagnetic wave that is propagating through the interaction region at approximately the same velocity as the moving electrons. The electrons give up energy to the electromagnetic wave through an exchange process characterized as electron beam interaction, which is evident by a reduced velocity of the electron beam from the interaction region.
These "spent" electrons pass out of the interaction region where they are incident upon and collected by a final element, termed the collector. The collector collects and returns the incident electrons to the voltage source. Much of the remaining energy in the charged particles is released in the form of heat when the particles strike a stationary element, such as the walls of the collector.
The electron collector can either be mounted directly to the body of the RF device containing the RF interaction structure, or can be electrically isolated from the structure. Isolated collectors are capable of operating at a significantly lower voltage than that of the RF device, and are known as depressed collectors. By operating the collector at a depressed state, the electric field within the collector slows the moving electrons so that the electrons can be collected at a reduced velocity. This method increases the electrical efficiency of the RF device as well as reducing undesirable heat generation within the collector. Depressed collectors are discussed in U.S. Pat. No. 4,794,303, by Hechtel et al., which is assigned to the same assignee as the present invention, and which is incorporated herein by reference.
A depressed collector typically comprises an outer metallic structure which is fixed to the RF device and forms part of the vacuum envelope of the interaction region. An inner metallic structure is centered within the outer structure, and serves as the recipient of the electron beam. These collector structures are often cylindrical shaped, but other alternative shapes are employed. To hold the inner structure in place, and to provide thermal conductivity and electrical isolation, standoff assemblies are provided which join the outer and inner structures. The standoff assemblies must provide for the conduction of heat from the inner structure to the outer structure, so that the heat can be ultimately removed from the device.
To provide the depressed electric field in the inner structure, a highly negative voltage is applied to the inner structure. Since the voltage of the outer structure is equivalent to that of the RF device, a voltage differential exists between the inner and outer collector structures, creating an electric field between the structures. The standoff assembly must be highly electrically insulative in order to prevent electrical conduction between the structures. If the voltage differential becomes too large, a breakdown condition can occur in which electrical arcing bridges across the surface of one or more of the standoff assemblies. This breakdown condition would significantly reduce the effectiveness of the depressed collector, and in some cases could damage the structure.
To provide the requisite electrical insulative quality, ceramic materials are typically used in the standoff assembly. These ceramic components can take a variety of forms, including solid sheets of ceramic material which partially or completely fill the field space, spheres which are uniformly arrayed inside the field space, and rectangular pads contoured to maximize the voltage standoff. However, these prior art standoff designs have met with less than desirable results due to the large voltages and thermal loads experienced with modern RF devices. The sheet ceramic designs are typically unable to handle high thermal loads without cracking. The sphere or pad shape designs are not able to hold off large voltage differentials without arcing. Thus the prior art standoff designs have been unable to achieve acceptable levels of both thermal conductivity and voltage breakdown resistance.
Therefore, it would be desirable to provide a highly depressed, conduction cooled collector having high thermal conduction capacity and voltage breakdown resistance. It would also be desirable to provide a depressed collector having a standoff design which combines a short thermal path through the standoff with a long voltage breakdown path across the surface of the standoff.